Fixes#10406
ImGui_ImplOpenGL3_Shutdown() calls imgl3wShutdown() which dlcloses the
GL library handles but does not zero out the imgl3w function pointer
table (imgl3wProcs). When a GLArea is re-realized (e.g. during
reparenting), ImGui_ImplOpenGL3_Init() calls ImGui_ImplOpenGL3_InitLoader()
which checks "if (glGetIntegerv == nullptr)". Since the stale pointers
are non-null, it skips re-initialization. The next GL call through a
dangling function pointer causes a SIGSEGV.
Fix this by introducing ImGui_ImplOpenGL3_ShutdownWithLoaderCleanup()
which calls the normal shutdown and then zeroes the imgl3wProcs table,
forcing the next Init to reload GL function pointers via imgl3wInit().
Also properly destroy the ImGui context and reset widget state in
glAreaUnrealize so re-realize starts clean. This was extra but was
probably leaking memory.
